1. Field of the Invention
The invention relates to a lens for an optical module of a motor vehicle and to an optical module of a motor vehicle comprising such a lens.
2. Description of the Related Art
It is known practice for a motor vehicle to be equipped with an optical device comprising several lighting modules intended to illuminate the road using various lighting beams providing low beam and/or high beam functions, as described hereinbelow:                In order to perform the low beam function, the optical device of a first vehicle needs to generate an optical beam that has a horizontal cutoff line mainly situated below the line of the horizon, in order to avoid dazzling the drivers of second, oncoming vehicles or vehicles in front of this first vehicle.        
To this end, it is known practice to provide the optical device with a shield and with a lens which are arranged in such a way as to generate this cutoff line, it being possible for the shield to be formed by a reflective horizontal surface also referred to as a beam bender.                In order to perform the high beam function, the optical device of a first vehicle needs to generate an optical beam that illuminates above the line of the horizon. In order to avoid dazzling the drivers of second, oncoming vehicles or vehicles in front of this first vehicle, the high beam needs to be deactivated when the first vehicle crosses with an oncoming vehicle or is following second vehicles.        
More recently, and in order to allow the driver of the first vehicle to have lateral visibility without dazzling the driver of a second vehicle, it has become known practice to use a system which automatically generates, within the lighting beam of the first vehicle, a shadow zone corresponding to the position of the second vehicle. Thus, this function, which will be referred to hereinafter as the selective high beam function, allows an optical device to illuminate on each side of detected second vehicles.
In order to illustrate the implementation of such a selective high beam function, FIG. 1 gives a 100 Isolux diagram of an optical beam performing this selective high beam function, i.e. comprising a shadow zone 102 corresponding to a detected vehicle 104, the sides of which are illuminated. In this example, the lighting beam in which the shadow zone 102 is generated is obtained using a low beam (curves 106 in continuous line) and two additional beams (curves 108 in dotted line) which are positioned in FIG. 1 on each side of the detected vehicle 104.
The present invention comprises the observation that while the creation of such a shadow zone 102 leads to the creation of vertical cutoff lines 110, the sharpness of these vertical cutoff lines 110 needs to be able to be adjusted differently from the sharpness of the horizontal cutoff lines 112 and 114 of the low beam and of the additional beam. In fact, the criteria for optimizing these cutoff lines as determined from actual testing appear to be very different, namely:                On the one hand, it is necessary for the sharpness of the horizontal cutoffs 112 and 114 to be degraded relatively sharply, notably when the cutoff lines of the two beams are achieved by means of a beam bender. Without this degradation, the cutoff lines specific to each beam are then highly pronounced (problem with contrast) and alternating (problem of homogeneity between contrasted zones) because it is difficult to achieve complete superposition of the horizontal cutoff lines.        
In order to obtain this relatively sharp degradation, it is known practice to provide the surface of a lens with microstructures that scatter the light in various directions, as described in the Holophane patent FR 2 925 656.
What is needed, therefore, is a lens and optical module that improves on the systems of the past.                On the other hand, it is necessary to maintain relatively good sharpness of the vertical cutoff lines 110 in order to ensure that the driver of the vehicle 104 situated in the shadow zone 102 is not dazzled by the additional beam.        